At the minimum, they should have had blowers ventilating that pit before they went in.
A friend of my wife’s just called from Chicago last Friday night.
Two guys working on her sewer crew (private firm) went down without gear (simply stupid) and died in the manhole.
People make this mistake constantly, in many industries.
When I was a plant supervisor, I enforced this (and lockout/tagout) with an iron hand.
Hydrogen Sulfide H2S, a deadly gas that need not displace oxygen to kill if you smell the “rotten egg” smell its a small amount when that smell goes away then you are in real trouble if you done nothing to disperse it. Its the same basic attitude most people have towards safety that got these farmers killed it takes training to change this mindset, and farmers especially have a mind set of irreducible ignorance that limits their behavioral changes. Its too bad for these people, but I would lay money on it that it was predictable.